The adult liver has a remarkable capacity to restore its mass in response to partial hepatectomy, liver transplantation, or toxic or inflammatory injury. Regeneration after partial hepatectomy is impaired in CCAAT enhancer binding protein beta(C/EBP-beta) -/- mice and is associated with decreased hepatocyte DNA synthesis, prolonged hypoglycemia and reduced expression of growth-associated and cell cycle-associated genes, including cyclin E. Resistance to Fas-mediated apoptotic injury in C/EBP-beta -/- livers also links C/EBP-beta to the regulation of the initial liver injury response. Physiologic associations between C/EBP-alpha and beta and the retinoblastoma protein in other cell types and the finding that pRb phosphorylation is decreased in C/EBP-beta -/- livers after partial hepatectomy link C/EBP-beta with pRb phosphorylation, cyclin E activation and hepatocyte cell cycle progression. Phosphorylation of specific domains in the C/EBP-beta protein overrides the block to cell cycle progression associated with induction of unmodified C/EBP-beta in cell models, suggesting that posttranslational modifications of C/EBP-beta are also necessary for hepatocyte cell cycle progression in the regenerating liver. The Specific Aims of this proposal are (1) To identify the component(s) of the apoptotic pathway that are abnormally regulated in Fas-antibody treated C/EBP-beta -/- livers and to determine if C/EBP-beta is also involved in the regulation of TNF-alpha mediated apoptotic liver injury. (2) To determine whether C/EBP-beta and/or C/EBP-alpha interactions with pRb are important for pRb phosphorylation, cyclin E activation and hepatocyte cell cycle progression. (3) To define the C/EBP-beta functional domains that are required for hepatocyte proliferation. C/EBP-beta proteins modified at residues that correspond to phosphoacceptor sites linked to signaling pathways in the regenerating liver will be introduced into CIEBP-beta -/- primary hepatocytes and assessed for their ability to facilitate hepatocyte cell cycle progression. Together these studies will provide important mechanistic insights regarding the basis for the response of the liver to injury and for hepatocyte proliferation following growth stimulation. This knowledge will be useful for the development of therapeutics to augment the regenerative capacity of the liver in a variety of liver diseases.